Back on topic -
Based on my current observations, I'd say that best way to train troops depends on your army composition and faction.
IMO, with the army I usually use (mostly Rhodok sharpshooters) there is no need for any Trainer skills in the party. It would help, yes, but points can be better invested. The reason for this is very simple - sharpshooters are lower tier than marksmen for instance and many other troops. Also, only Recruits are really bad, but next tier is already crossbowman which is good enough. That crossbowman will gain experience very fast in combat, with very low risk of getting killed (of course, careful picking of battles is necessary). On the other hand, getting Sergeants without Trainer is more difficult because melee troops need to enter melee to gain more experience points, and weak melee units (lower tier) have much higher chance of getting killed.